1. Field of the Invention
The invention relates to processing semiconductor crystal rods and somewhat more particularly to a method and apparatus for crucible-free zone melt processing of a semiconductor crystal rod whereby formation of dislocations and irregularities within the process crystal are avoided.
2. Prior Art
Semiconductor crystals, particularly those composed of silicon, are generally produced by a crucible-free zone melt process whereby a monocrystalline seed or nucleation crystal having a relatively small diameter is melt-connected, as with the aid of an induction into coil, to an end of a relatively large diameter polycrystalline semiconductor member and a melt zone is generated at the junction of the seed crystal and the polycrystalline member and passed one or more times along the length of the polycrystalline member. The melt zone is moved by providing relative movement between the polycrystalline member and a heat source, such as an induction heating coil, which may be the same one used to melt-connect the seed crystal with the polycrystalline member or be different therefrom. In this manner, a polycrystalline member is purified of any foreign constituencies and simultaneously converted inot a monocrystalline member.
In the production of semiconductor components from so-produced semiconductor materials, it is desirable that the semiconductor material be as free as possible from dislocations and other irregularities which materially interfere with the electrical properties of the semiconductor components produced therefrom. Further, the presence of dislocations, etc. within the semiconductor material decreases the life of minority carriers within such semiconductor materials.
German Auslegeschrift No. 1,079,593 (which generally corresponds to British Letters Patent No. 889,160) suggests that dislocations in rod-shaped semiconductor members may be decreased at the melt-connected juncture of the seed crystal and such semiconductor members by decreasing the cross-section of the semiconductor member at the direct proximity of such melt-connected juncture prior to the last pass of the melt zone through the semiconductor member. Dislocations which may be present in the seed crystal are thus given a chance to heal in the thus-produced thin connecting piece or bridge between the seed crystal and the semiconductor member.
German Letters Patent No. 1,128,413 (which generally corresponds to U.S. Pat. No. 3,175,891) discloses that substantially dislocation-free rod-shaped silicon monocrystals may be produced, for example, by controlling the rate or speed of travel of one or more passages of a crucible-free zone melt through the rod. This reference suggests that all passages of the melt zone start in the seed crystal and that the travel speed of the melt zone in the seed crystal should be controlled so as to be in the range of about 7 to 15 mm/min. During the last pass of the melt zone, the silicon rod cross-section at the juncture of the seed crystal and the silicon rod is constricted by a temporary relative movement of the rod ends at a speed greater than 25 mm/min. while the speed of the melt zone is steadily decreased from this constriction point until the full cross-section of the rod is attained. Thereafter, the melt zone is moved through the rod at a speed less than about 7 mm/min.
It has been noted when monocrystalline semiconductor rods of a fairly large diameter are being produced by the crucible-free zone melt process, the rod-shaped monocrystals which grow at the seed crystal at the last passage of the melt zone tend to oscillate, particularly at the thin-connecting or bridging piece between the monocrystal and the seed crystal. This drawback is particularly acute when thick monocrystalline semiconductor rods are being produced, for example, by way of shortening or compacting during a crucible-free zone melt process. These oscillations appear to cause the development of dislocations and other irregularities in monocrystals of the material as it becomes rigid during the last passage of the melt zone through the semiconductor rod. In addition, such oscillations often cause a dripping of molten material from the melt zone or even a breakage of the thin-connecting or bridging piece between the seed crystal and the semiconductor rod, which, of course, causes an interruption in the melt zone process.
I have disclosed in German Offenlegungsschrift No. 1,519,901 a means of supporting the ends of a crystal rod at the juncture thereof with a seed crystal which comprises a finger-like support means that is positioned on the upper edge of a casing and which is axially movable and encloses the mounting for the seed crystal. However, this arrangement does not completely obviate vibrations or oscillations during the growth of very thick (i.e. having a diameter larger than 30 mm) dislocation-free semiconductor monocrystalline rods since the finger-like supports do not uniformly touch the overall round cone portion of such rod. Due to this instability, increased oscillations may be produced opposing the supporting effect desired or may even eliminate any beneficial supporting action.